46824-GB6
Theoretical Investigations of Transition Metal Carbides
Transition metal carbides (TMCs) have received considerable attentions from experimentalists and theoreticians because of their unique physical and chemical properties and exhibit advantages over their parent metals in terms of activity, selectivity, and durability. Our understanding of TMCs is very limited, both experimentally and theoretically. The goal of this project is to supply the reliable theoretical results for the future experimental study. The theoretical challenge is the existence of partially filled d shell of the transition metal atoms leads to the high density of low-lying atomic states. Multi-reference methods with a big active space are usually required. However this approach is always restricted by the size of the active space. The tradition way is to freeze the single and double excitations from the certain orbitals in the active space.
The compound we started is NiC2. Our first step focused on NiC. The CASSCF and the subsequent MRPT (based on the complete active space) calculations were performed using both small (3d and 4s on Ni and 2s and 2p on O) and big (3s, 3p, 3d, 4s on Ni and 2s and 2p on O) active spaces. From CASSCF calculations, it showed that the occupation on 3s and 3p are almost same as when they are included in the frozen core. But the existence of 3s and 3p orbitals from Ni is important in describing Ni-C bond and then improves the quality of the reference. The subsequent MRPT calculations, using the 2nd order Generalized Van Vleck Perturbation Theory, showed that results obtained using the big active space are in better agreement with experimental data. The second step is to extend this method to NiO2, an analog of NiC2, which also puzzles many experimental and theoretical chemists, in order to determine how to truncate the active space. The results demonstrated that, after including 3s and 3p of Ni in the active space, the complete reference for the MRPT does not improve the results. Therefore, the MCSCF based MRPT can be used after the CAS-type reference is done.
Current studies focus on the ground state and low-lying excited states of NiC2. As expected, the splitting energy may not be very big and more interactions have to be considered. For the Ni system, it is known the scalar relativistic effect and spin-orbit coupling may be important and have to be corrected in order to get the accurate data. SA-CASSCF and Douglas-Kroll-Hess (DKH) are applied. If this work is successfully solved, the ground state and low-lying excited states of NiC2 can be determined and the bonding characteristics can be identified. And the similar investigations can be extended to other similar systems.
